[BUGFIX] Use absolute path to check for extensions

The functional test utility assumes that the current
working directory is the document root, which must
not necessarily be true.

Use the absolute path to check for existence of
extension directories instead.

Resolves: #56204
Releases: 6.2
Change-Id: I9e5fa06f0e069c04586de7c23edc80e070ef4aa9
Reviewed-on: https://review.typo3.org/27786
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel